The present invention relates to eddy current measurements of an object during manufacturing, and more particularly, flaw detection and characterization of a nuclear fuel tube having an irregular axial motion.
Zircaloy tubing is produced by cyclic cold working impacts in pilger mils to reduce the tube diameter. Thereafter additional steps are performed, e.g., annealing to relieve stress, etching to obtain a smooth outer surface, straightening, and polishing to remove small surface flaws. Finally, an "offline", i.e., after the total production process at a location separate from the process machines, eddy current flaw detector is used to reject tubes having flaws produced by the diameter reducing step. Because of the time required by the intermediate process, the pilger mills may produce a large quantity of defective tube before the problem is detected by the "offline" inspection. This delay in obtaining feedback can result in lost productivity and costly scrap. The delayed "offline" inspection is customarily employed because it is technically simple to implement.
For example, consider the eddy current signal due to a point defect on e tube which passes through a differential eddy current sensor, i.e., a pair of coils disposed about the tube at different axial positions and connected in a bridge circuit such that the impedance difference between the two coils is sensed. As the tube advances, the defect first passes through one coil of the differential eddy current sensor and drives the sensor output signal positive. In an offline scanner that moves the tube at a steady velocity, the same flaw passes through the second coil of the differential sensor a precise time later and it drives the output signal negative. The net effect of the passage of the defect is the sensor output signal having a single cycle of an updown waveform with a temporal period that can be calculated from the tube velocity and coil separation. This signal has a strong center frequency which can be easily enhanced by frequency domain filtering of the output voltage. Moreover, since the scanner moves all the tubes at a uniform velocity, all small flaws will have approximately this period. This technique can be equally well applied to other eddy current coil configurations.
It will be appreciated that it is desirable to inspect the tubing for flaws right after the diameter reducing step ("online") both to save the expense of the later steps for the inspected tube if it is flawed and toe able to immediately taken action to reduce the number of flaws in later processed tubes. However, the finished tube emerges from the pilger mill in uneven amounts due to an irregular axial motion, sometimes actually moving backward for short distances. In addition, mechanical coupling to the mill makes the tube vibrate perpendicular to its long axis. Moreover, the tube rotates irregularly about its axis and may also be bowed so that it rotates about the axis of the eddy current coils. All of these irregular motions result in artifacts in conventionally acquired eddy current data.
Two of these potential error sources can be handled by conventional engineering. Careful fixturing can reduce the effect of vibrations to a manageable level, and can reduce the effect of bowed tubing. Other rotational motion has no effect on the eddy current signal, due to the cylindrical symmetry of both the tubing and the encircling eddy current coil. However, fixturing cannot control the irregular axial motion of the product, and online scanning remains difficult at best.
In particular, the flaw will first drive the signal positive as it encounters the first coil of the differential sensor. The time lapse before the occurrence of the negative-going signal as the flaw passes through the second coil is now indeterminate. In fact, retrograde motion could even cause a section of tubing with a flaw to pass through the first coil several times before the second half of the sensor is reached. Thus the eddy current output signal, which is sampled at even time intervals, will be complicated by a motion artifact that no amount of careful fixturing will remove. The collection of useful signals with eddy current sensors mounted directly on the production mills is precluded by the erratic motion of the tubes during the diameter reducing pilger cycle. The tube motion also lowers the signal-to-noise ratio of the output signal.
It is therefore an object of the present invention to provide apparatus and method for making accurate measurements with a high signal-to-noise ratio on an irregularly moving object and to reduce the cost and increase the quality of the object when being processed.